1. Field of Invention
Embodiments of the invention are directed to inks for use in ink jet printing. More in particular, low viscosity, substantially solvent-free inks that are curable by free radical initiation upon exposure to ultraviolet (UV) light are described.
2. Description of Related Art
The volume of digital color printing is expected to experience significant growth in the coming years. The color images provided by ink jet printing inks are overwhelmingly preferred in panel studies over other digital imaging systems. There is also a strong case to be made that the total cost of ownership of an ink jet printer will ultimately be cheaper than similar volume electrophotography units.
Ink jetting devices are known in the art, and thus extensive description of such devices is not required herein. As described in U.S. Pat. No. 6,547,380, incorporated herein by reference, ink jet printing systems are generally of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. There are three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure that it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In a typical design of a piezoelectric ink jet device, the image is applied by jetting appropriately colored inks during four to six rotations (incremental movements) of a substrate (an image receiving member or intermediate member) with respect to the ink jetting head, i.e., there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration.
Piezoelectric ink jet devices typically suffer from two shortcomings. First, the images formed are presently not very robust. Second, the printers consume large amounts of energy.
Hot melt inks typically used with ink jet printers of the aforementioned type utilize a wax based ink vehicle, e.g., a crystalline wax ink. Such solid ink jet inks provide vivid color images. However, the use of crystalline waxes also has at least two shortcomings. First, the printhead must be kept at least at 135° C. throughout printing with the device. The wax based inks are heated to such high temperatures to decrease their viscosity for proper jetting. Moreover, if the printhead is cooled and re-warmed, a lengthy purge cycle that consumes significant amounts of ink must be carried out. Second, the brittle crystalline waxes do not provide robust images and are easily scratched. This is because wax based inks generally crystallize at temperatures greater than room temperature and therefore, the wax based ink that has been transferred to the recording medium is essentially as hard as it will get. The high energy consumption, waste of expensive ink during purging, and fragile images all cause customer dissatisfaction, and in some markets prevent any sales penetration at all.